1. Field of the Invention
The present invention relates to fluid vibrating type flowmeters for measuring flow rates of various fluids (gases and liquids), such as gas meters, and more particularly to a fluid vibrating type flowmeter including a nozzle mounted in a flow passage and defining an outlet plane extending perpendicular to the flow passage, an enlarged passage continuous with a nozzle outlet and having diverging passage walls symmetric about an axis of the nozzle, a target disposed centrally of the enlarged passage for blocking straight movement of streams jetting out of the nozzle, and a constricted passage disposed downstream of the enlarged passage and having a smaller passage width than the enlarged passage.
2. Description of the Related Art
Fluid vibrating type flowmeters of the type noted above which have been proposed heretofore include a flowmeter as shown in FIG. 12. The operating principle of this fluid vibrating type flowmeter will be described briefly first. Streams jetting out through a nozzle outlet plane 11 are divided into main jet streams L1 flowing round a target 20 and out through a constricted passage, and return steam L2 branching from the main jet streams L1 and impinging upon downstream positions of the enlarged passage or a narrowing section defining the constricted passage to flow backward through the flow passage. With this type of flowmeter, when fluid jets out of the nozzle, the jet streams are drawn by Coanda effect to one of side walls 50 and 51 extending along the flowing direction. That is, the jet streams are diverted to either side wall 50 or 51 instead of flowing straight. Then, the return flows L2 are generated which impart a fluid energy in a direction perpendicular to the straight flowing direction of the jet streams in the vicinity of the nozzle outlet plane. As a result, the jet streams will in the next step flow along the opposite side wall 50 or 51. Thus, the return flows L2 act as control flows for controlling the jet streams in the vicinity of the nozzle outlet, whereby the streams jetting out of the nozzle flow alternately along opposite sides of the target (the presence of the target effectively induces vibrations on a low flow rate side). With the flowmeter having only the target in the enlarged passage as shown in FIG. 12, vortices formed downstream of the target also influence such vibrations. Generally, the cycles of vibrations are proportional to the rate of fluid flow through the flowmeter. The rate of fluid flow through the passage is measured by using this phenomenon.
In the flowmeter including the enlarged passage having a substantially box-like configuration as shown in FIG. 12, mechanisms for detecting pressure or flow rate are arranged in a pair of measuring positions 55 immediately downstream of the nozzle outlet plane and opposed to each other across the jet streams. Flow rate is detected by measuring the number of vibrations based on the pressure differences or flow rate variations resulting from the phenomenon of the jet streams flowing alternately along the opposite sides of the target.
In the case of a gas meter, for example, allowable measurement errors (differences between an actual flow rate and detection values provided by a measuring device) generally are .+-.2.5% for a range of flow rates from 0.15 to 0.6 m.sup.3 per hour, and .+-.1.5% for a range of flow rates from 0.6 to 3 m.sup.3 per hour (which are shown in broken lines in FIG. 13). When measurement is taken with a flowmeter including an enlarged passage having a box-like configuration as shown in FIG. 12, errors occur as shown in a solid line in FIG. 13. FIG. 13 shows measurement errors (%) occurring with respect to correct detection values when flow rate is varied (0.1 to 5 m.sup.3 per hour). Such errors are hereinafter referred to as flow rate/device difference characteristics. In this measurement, the errors occurring for a very low flow rate range (0.15 to 0.4 m.sup.3 per hour) are .+-.4.4% far exceeding the allowable measurement criterion, and measurement values meeting the allowable measurement criterion are obtained only for the range of 0.4 to 2.1 m.sup.3 per hour. The values shown as .DELTA.E in the drawings are values showing Emax (maximum value on the positive side-Emin (maximum value on the negative side) in flow rate/device difference characteristics, which form the basis for determining measurement stability. (In the embodiments and experiments discussed hereinafter, all tests of flow rate/device difference characteristics of flowmeters are carried out as in the above example, i.e. using air as a gas and covering a flow rate range up to 5 m.sup.3 per hour. This is based on consideration of variations in Reynolds number occurring with other types of gases such as propane gas when measurement is taken for 3 m.sup.3 per hour which is an upper flow rate limit of the allowance criterion.)
According to the allowance criterion, such numeric values are 5% for a low flow rate range and 3% for a high flow rate range. That is, the above conventional construction cannot be employed in a measuring instrument, and the prior art has room for improvement with respect to measuring precision.
With a view to solving the above problem, various configurations are conceivable for fluid vibrating type flowmeters satisfying the allowance criterion. It is desirable that principal pertinent dimensions are geometrically standardized for manufacture of such flowmeters.